1. Field of the Invention
The present invention relates to the mining industry and, more particularly, to an improved vertical drainage system for a heap leach pile with vertical wicking conduits for draining trapped fluid and porewater down through the strata of a heap pile.
2. Description of the Background
Heap leaching is an industrial mining process to extract precious metals, copper, uranium, and other compounds from ore by percolation. Heap leaching is widely used across North America and the world.
In conventional heap leaching operations a base area of ground is prepared and graded into a sloping foundation, normally a very large area (e.g., 2,500 feet by 1,200 feet), and an impermeable membrane is placed atop the foundation. Some form of collection system is placed on top of the impermeable membrane to collect the residual product. The collection system or “pad” minimally entails a granular cover fill supplemented with perforated drain pipes spaced throughout to collect and transport the operational leach flows. More complex multi-layer heap leach collection pads are also common. Crushed ore is normally heaped onto the collection pad to a depth to 10 to 30 feet and leveled off at the top of the heap. The ore is typically pre-crushed to a desired size. After the foregoing “heap” has been prepared, the target metal is leached from the ore by circulating a leaching solution down through the heap (in a process commonly known as “percolation”). To effect percolation, the solution is distributed over the top of the heap and permitted to seep down through the heap to the collection pad and drain pipes. There are various options for distributing the leaching solution. For example, U.S. Pat. No. 5,005,806 to Krauth discloses a drip system that includes mainline pipes extending longitudinally along a side of a heap, parallel header pipes extending at spaced intervals from the main pipeline across the heap, and parallel, closely spaced tubes extending longitudinally between the header pipes, each tube having a plurality of outlets for leaching solution.
Percolation separates the valuable fraction from the uneconomic fraction of ore, and the metal-laden product solution (called the “pregnant solution”) is recovered from the drain pipes. Metals, such as gold, silver or copper, can be recovered from the pregnant solution in a conventional manner, such as by precipitation with zinc dust or aluminum powder, carbon absorption or by electrowinning.
The residual materials left over after the heap leaching process are called “tailings” or “tailings piles” (e.g., mine dumps, culm dumps, slimes, tails, refuse, leach residue or slickens).
Unfortunately leach heaps and tailings piles are large, tall, and have a significant slope or geotechnical failure potential. In the last four decades the mining industry has experienced numerous fatal slope failures or “flowslides.” One cause of slope failure is that the solution can accumulate in the tailings pile and become trapped at various elevations. Over time the fraction of leaching solution trapped in the particulate increases. This not only reduces effectiveness of the heap leach pile, but eventually causes geotechnical instability in the piles. Ultimately if not managed the buildup of these fluids can cause a devastating flowslide.
Accordingly, an important object of the present invention is to provide a drainage system for heap leach piles to drain buildup of fluid within the heap pile and avoid geotechnical or slope failure.